Alcoholics Anonymous (AA) is the most popular self-help program for alcohol-related problems, and 12-step therapy, based on AA doctrine and practice, is the prevailing alcohol treatment model in the United States (Morgenstern et al. 2002). Studies of 12-step therapy indicate that it is as effective as therapies that are more research supported, such as cognitive-behavioral approaches, and that it may actually be superior when total abstinence is the goal (Ouimette et al. 1997; Project MATCH Research Group 1997, 1998). A primary objective of 12-step therapy is to facilitate AA participation, and evidence suggests that this aim contributes to its overall success (e.g., Moos and Moos 2005; Tonigan 2005). To date, significant gains have been made both in identifying the individual characteristics predictive of AA affiliation (e.g., Emrick et al. 1993; Tonigan et al. 1996, 2006) and in developing effective clinical strategies to promote AA affiliation (e.g., Nowinski 2006; Nowinski et al. 1992). Furthermore, we now have a clearer picture regarding the nature and magnitude of benefits associated with AA attendance (e.g., Gossop et al. 2003; Kelly et al. 2006; Tonigan 2001). Understandably, then, high priority now has been assigned to investigation of what actually occurs in AA, with a special focus on identifying how prescribed AA behaviors and beliefs mobilize drinking reduction. This chapter focuses on the psychological mechanisms explaining how and why AA is beneficial for many substance abusers. Our goal is to provide readers with an evidence-based review of what is currently understood about AA-related change and to highlight some associated clinical recommendations.